Process and apparatus for fluid treatment of pervious sheet materials



Feb. 15, 1955 A. E. UsHAKoFF 2,702,229 PROCESS AND APPARATUS FOR FLUID TREATMENT OF PERVIOUS SHEET MATERIALS Filed June 27, 1951 e UAUAVAUAVAV Fig. 2

Afm?, fm 7, ma JMJ! ATTORNEYS INVENTOR. ALEXIS E. USHAKOFF PROCESS AND APPARATUS FOR FLUID TREAT- MENT OF PERVIOUS SHEET MATERIALS Alexis E. Ushakoff, Beverly, Mass., assignor to Secotan, Inc., Somerville, Mass., a corporation of Massachusetts Application June 27, 1951, Serial No. 233,924

17 Claims. (Cl. 8-150.5)

This invention relates to the treatment of animal skins and similar pervious sheet materials and in particular to a process of forcing fluids through such sheets while supported on a porous bed. Apparatus for carrying out this process is also included within the scope of this invention. W.

It is frequently necessary that sheet materials be treated throughout with fluids, and it has been long recognized that it is advantageous to force these fluids through such materials to accelerate the treatment. ln particular, for instance, a skin may be tanned much faster if the tanning solution is forced through the skin under a positive pressure, than it can if the tanning agent is caused to permeate the skin solely by diffusion processes. This is especially true if tanning is carried out under non-aqueous conditions, as by the process described in the example below.

An important limitation on the utility of processes of forcing fluids through skins is imposed by the variations in the size and shapes of different skins. The area over which pressure is applied must be either occupied solely by skin or rendered impervious where not so occupied. In general in the processes of the prior art either selected lregions of a skin are separately treated or the skin is trimmed to be coterminous with the area over which pressure is applied. These expedients are restorted to to prevent the liquid from channeling around the skin, rather than through it.

This invention provides means whereby one piece of equipment, containing a porous bed over which pressure is applied and on which a skin is laid, can accommodate any skin capable of being placed on the bed, regardless of whether the entire bed is covered by it. Moreover, any skin may be so treated without making any special adaptations of the equipment dependent on the size and shape of the skin; by this invention any skin may be accommodated for treatment by fluids under pressure by techniques independent of the size or shape of the skin.

According to the present invention a bed of porous material and means for forcing a fluid through it are provided, and a skin, not necessarily coterminous with the bed, is laid upon its upper surface. An impervious membrane of thin flexible and preferably somewhat elastic material is thenllaid over the entire bed and a fluid pressure differential is applied across the bed and membrane to force the membrane against the bed. The fluid with which the skin is to be treated is then introduced between the skin and the membrane under a pressure and at a rate suflcient to cover the skin but insufllcient to separate the membrane from the parts of the bed not covered by the skin. During the introduction of this fluid to the skin the fluid pressure on the membrane is maintained to keep the membrane in contact with the bed, thus preventing this fluid from flowing in substantial amounts around the skin, through the regions of the bed which are not covered by the skin.

A feature of this invention is that the membrane is caused to seal selectively those regions of the bed which are not covered by the skin without sealing against the skin. If, for any reason, the membrane should be separated from the bed, the fluid will flow through this uncovered region for only a short time before the membrane is again pressed tightly against that part of the bed, thus resealing it. lt has been found that a criterion for selective sealing by the membrane of those parts of a 1A" wool felt bed which are not covered by the skin is a fluid pressure drop through the skin of at least 2 p. s. i.

nited States Patent O 2,702,229 Patented Feb. 15, 1955 ICC Inasmuch as it is only necessary that a fluid pressure exist above the membrane and that the liquid with which the skin is to be treated be introduced on the skin between the skin and the membrane, the fluid introduced above the membrane to seal it selectively may be the fluid with which the skin is to be treated, and this fluid may be caused to flow onto the skin by providing a hole in the membrane above the skin. Although initially, before conditions satisfactory for sealing are established, the fluid flows directly through the bed, the parts of the bed which are not covered by the skin become sealed quickly and the fluid is thereafter free to flow predominately only through the skin. To assure that this fluid distributes itself over the entire skin, means are preferably utilized to support the membrane away from the skin thus to prevent the membrane from falling against the skin and impeding the flow of this liquid over it. It has been discovered that this may be done without causing the membrane to be pulled away from those parts of the bed which are not covered by the skin.

In another embodiment of this invention separate means are provided for applying a fluid pressure above the membrane and for introducing the desired fluid between the skin and membrane. In this embodiment a convenient method of preventing the membrane from falling against the skin is to introduce the desired fluid onto the skin under a slightly higher pressure than exists above the membrane. ItA has been discovered that this may be done without causing the membrane to lift from those parts of the bed adjacent the skin which are properly sealed by the membrane.

Preferred representative embodiments of this invention, selected for purposes of illustration, are described in detail below wherein reference is made to the accompanying drawings, in which:

Fig. 1 is an elevation view in cross section of apparatus for practicing this invention;

Fig. 2 is a plan view, partly broken away, of the apparatus shown in Fig. l;

Figs. 3 and 4 show, in cross sectional elevation, a fragmentary section of a skin and membrane on a porous bed during successive stages of the process of this invention;

Fig. 5 is an elevation view in cross section of alternative apparatus for practicing this invention; and

Fig. 6 is an elevation view in cross section showing optional modifcations of the apparatus shown in Fig. 5.

A simple apparatus with which this invention may be practiced is shown in Figs. l and 2. A base member 1 is provided having a raised peripheral edge 2 with a flat top surface, and a drain connection 3. On the base member 1 and within its peripheral edge 2 is a supporting screen 4 and on this is a porous bed 5. The top surface of the bed 5 is preferably approximately level with the top of the peripheral edge 2.

The porous bed 5 may consist of any porous material more or less readily permeated by the liquid with which the skin is to be treated. Wool felt, about Mi thick is satisfactory. Plaster of Paris and other porous ceramic materials or woven fabrics and screens may also be used satisfactorily. The supporting screen 4 serves to support the bed 5 and provides in its interstices a manifold region coextensive with the bed.

To practice the process of this invention a skin 10 which has been pickled or bated is laid flat upon the bed` 5, and a spacer 11 is placed on the skin. The spacer 11 may consist of a ring having downwardly extending legs 12 which support it above the skin.

Over the bed, skin and spacer is now placed a thin membrane 15 of a flexible preferably somewhat elastic material. The membrane is provided with a hole 16 through which the desired liquid may be poured onto the skin. The hole 16 is placed above the spacer 11 and over the skin 10. A rubber dam about 3 mils thick is a satisfactory membrane, but alternatively membranes of polyethylene, polyvinyl alcohol and similar synthetic organic polymers may be used. In any event a membrane should be selected which will not be attacked by the liquid with which the skin is to be treated. The margins of the membrane 15 should extend onto the edge 2 of the base member 1 so that the entire bed 5 is covered by membrane. The margins of the membrane 15 may lie free on the edge 2, but preferably a rim member 17 is placed over the margins of the membrane on the edge 2 to secure them and prevent their rufliing.

A pressure differential is now applied across the bed, with the membrane 15 on the high pressure side. This is conveniently done by drawing a vacuum on the drain connection 3. The membrane is thereby drawn tightly against the bed and also, to a lesser extent, against the skin. The uid with which the skin is to be treated is now introduced onto the skin through the hole 16. It ows through the spacer 11, under the membrane and over the surface of the skin to the edges thereof, and is drawn through the skin by the vacuum applied beneath the bed. Meanwhile the membrane covering those portions of the bed which are not covered by the skin is held tightly against the bed and prevents any substantial ow of liquid through the bed beyond the skin.

The best manner that has yet been found of carrying out the process of this invention utilizes apparatus shown in Fig. 5. The bed 20 of 1/1," wool felt is supported on a 10 mesh Wire screen 21 which lies on a base plate 22. A cover 23 having a reinforcing arched top 24 is hinged to overlie the bed 20 and base plate 22. A frame 25 surrounds the bed 20 and screen 21 and is situated between the base plate 22 and cover 23. The top and bottom surfaces of the frame 25 are provided with sealing gaskets 26 and 27 to effect a tight seat between the frame and both the base plate 22 and the cover 23. A drain connection 28 is provided in the base plate. A pipe 29 enters through the cover 23 at about its center and a valved air vent 30 also enters through the cover 23. Bolts 31 pass through the base plate 22, frame 25 and cover 23 to provide means for clamping the apparatus tightly closed.

In the operation of this apparatus a skin, preferably pickled or bated, is placed on the bed 20 and a spacer 11 is placed on it to lie under the opening of the pipe 29 when the cover 23 is down. This spacer serves to support the mid-section of the membrane above the skin so that the desired liquid may be introduced between the membrane and skin. It is contemplated that if a buoyant membrane which will float on the liquid is used, no separate spacing means are necessary. A rubber darn about 3 mils thick, having a central opening 16, is placed over the bed, membrane, and spacer with its marginal edges overlying the upper gasket 26 of the frame 25, and with its central opening 16 underlying the pipe 29 when the cover 23 is down. The cover 23 is now lowered and the bolts 31 are inserted and tightened.

The liquid with which the skin is to be treated is now introduced through the pipe 29 under pressure, while air is vented through the valved pipe 30 until the entire space above the bed is filled with the liquid. Upon the continued introduction of this liquid the membrane 15 seals those portions of the bed 20 which are not covered by the skin 10, and the liquid is thereafter caused to iiow substantially solely through the skin.

Alternatively air, instead of the liquid, may be introduced through the valved pipe 30, or a similar pipe, to cause the membrane 15 to seal the uncovered parts of the bed 20, and thereafter the liquid may be introduced onto the skin 10 through the pipe 29, entering through the opening 16 in the membrane, and iiowing over the skin. The air pressure over the membrane, urging the membrane against the bed, should be maintained while the liquid is thus owed onto the skin.

Modified forms of the apparatus just described and shown in Fig. 5 may also be used to practice the process of this invention. One modified apparatus is shown in Fig. 6. It is identical with the apparatus shown in Fig. 5 except that the membrane 15 is secured to the cover 23 about the inlet pipe 29, with the hole 16 surrounding the opening of the pipe 29. A ring 34, screwed to the cover 23 about the opening of the pipe 29, may conveniently be used to secure the membrane 15 in this fashion. In this apparatus the desired liquid is introduced directly onto the skin 10 under the membrane 15 through the pipe 29 while a separate fiuid under a separately controllable pressure is introduced above the membrane 15 through the pipe 30. To control and maintain the proper pressure differential between the pressures of the two iiuids, a pressure differential valve 33 is provided in the pipe 30 with actuating pressure takeolf lines 33a and 33h communicating reSDeCiVely With the pipe 30 and the pipe 29.

In operating this apparatus the skin 10 and membrane 15 are placed over the bed 20, as explained above, and the cover 23 is closed and tightened by the bolts 31. No spacer is placed on the skin to support the membrane. The desired liquid is introduced on the skin through the pipe 29 and the air or some other uid is introduced through the pipe 30 under a pressure sutlicient to eiect the desired selective sealing of the bed where it is not covered by the skin. This sealing of the bed can be detected by a decrease in the rate of ow of the liquid from the outlet 28, at which time the liquid ows only through the skin. Thereafter the air pressure above the membrane is maintained slightly below the pressure of the liquid on the skin below the membrane. This pressure difference need not be more than a few centimeters of water, and should not be so great as to allow the membrane to lift from those parts of the bed which it seals, as would be evidenced by a sudden abnormal increase in liquid flow from the outlet 28. The pressure differential valve 33 may be adjusted to maintain this desired differential, and in adjusting the valve account should be taken of the height of the valve above the bed and the consequent difference in pressure of both fluids, especially the liquid, between the valve and the bed. By so maintaining the liquid under a slightly greater pressure than the uid above the membrane, the liquid is caused to ow over the skin and prevents the membrane from falling against the skin and disturbing this flow.

It will be understood that the above description of the manner of practicing the process of this invention in the apparatus just described is but one manner of using that apparatus, and that experience will provide simplified routines. For instance, the air pressure may be applied before any liquid is introduced onto the skin, and a skilled operator, by observing various pressures, as above and below the membrane, will be able to determine when the bed is properly selectively sealed.

It will be further understood that the above description of various embodiments of the process and apparatus of this invention are only by Way of illustration. It is contemplated that modifications will occur to those skilled in the art, and that such modifications may be made without departing from the spirit and scope of this invention as broadly described above.

Figs. 3 and 4 diagrammatically show the skin and membrane during the successive stages of the process of this invention. In Fig. 3 the skin 10 and membrane 15 are shown lying limply on the bed 5 prior to the establishment of a pressure differential across the bed. Fluid is free to ow initially through the entire bed. However, after a pressure differential is applied across the bed the membrane is forced tightly against those regions which are not covered by the skin. Thereafter the membrane may be supported away from the skin, as by its buoyancy, by mechanical means, or by the creation of a fluid pressure under the membrane very slightly in excess of that above the membrane, without causing any substantial iiow of iiuid through the bed where it is not covered by the skin. The liquid with which the skin is to be treated may then be flowed under the membrane and over the skin and forced through the skin by the pressure differential across the bed, as shown in Fig. 4. It appears that the bed which is not covered by the skin becomes selectively sealed by the membrane as a result of the faster ow of uid through those parts of the bed. It has been found that for successful selecting sealing of a 1A wool felt bed there must exist a difference between the ow rates through the bed alone and through the bed and skin together represented by a pressure drop of at least 2 p. s. i. across the skin alone, otherwise the diierence in ow between those parts of the bed which are and those which are not covered by the skin is not sufficient for selective sealing. After the bed is properly selectively sealed such a ow rate need not be maintained, but should for any reason the membrane lift from the bed, these ow conditions are again necessary to reseal the bed.

It is believed that selective sealing is the effect of a lowering of the pressure, by the faster flow of fluid, according to Bernoullis principle, in the regions of the bed which are not covered by the skin, and that resealing, if part of the membrane should lift from the bed, is similarly effected. It is further believed that after the membrane has sealed those parts of the bed which are not covered by the skin the entire pressure drop falls across such parts of the membrane to force it against the bed, and the membrane may be selectively supported away from the skin without lifting it away from the bed because of the greater pressure urging the membrane against the bed than against the skin. Thus a liquid may be successfully introduced onto the skin and forced through it by the pressure differential across the bed.

The foregoing hypothesis is presented as a possible explanation of the manner in which the process of this invention is effective, and not as a definition of this invention. That other hypotheses may be offered and prove more correct should in no way limit the scope of this invention.

To tan a skin according to the process of this invention the skin is placed on the bed and covered by the membrane, and the membrane is then caused to seal those parts of the bed which are not covered by the skin, all as described above. Acetone is then introduced on the skin and forced through it by the effect of the pressure differential across the bed. The flow of acetone through the skin is continued until the eflluent acetone has a density no greater than .810 C.), which condition is reached when the skin is substantially dehydrated. Thereafter a non-aqueous tanning agent is introduced on the skin and forced through it until the skin is tanned. One such agent consists of a neutralized solution of chromic chloride in methanol and acetone, prepared by mixing 100 parts by volume of a saturated solution of chromic chloride in methanol with 500 parts by volume of acetone. A saturated solution of ammonia in water is added to this mixture to the point of incipient precipitation.

Initially the effluent tanning solution from this process is substantially colorless, because the skin adsorbs substantially all the chrome during its tanning. Tanning may be determined to be complete when the characteristic green of the chromic chloride again appears in this eflluent liquid.

By this process no special adaptation or preliminary steps are imposed by the shape or size of the skin. The routine for each skin is substantially identical and only a few minutes are required for its tannage. It will thus be seen that the application of this invention to the tanning art alone represents an important advancemet in that art.

Although this invention has been described with special reference to the treatment of skins, particularly to their tanning, it will be understood that the process herein disclosed may also be applied to the fluid treatment of other permeable sheet materials.

Having thus disclosed my invention and described in detail preferred embodiments thereof, I claim and desire to secure by Letters Patent:

l. Apparatus for treating pervious sheets with fluids under pressure comprising a porous bed capable of supporting the sheet and having a substantially flat upper surface, a flexible impervious membrane for covering said bed, means for applying and maintaining a fluid pressure differential across the bed and membrane to urge the membrane against the bed, and means for introducing the desired fluid between the membrane and a sheet on the bed.

2. Apparatus for treating pervious sheets with fluids under pressure comprising a porous bed capable of supporting the sheet, a flexible impervious membrane for covering said bed, said membrane having a hole adapted to overlie a sheet on the bed, means for applying and maintaining a fluid pressure differential across the bed and membrane to urge the membrane against the bed, and means for introducing the desired fluid through said hole and between the membrane and a sheet on the bed.

3. Apparatus for treating pervious sheets with fluids under pressure comprising a porous bed capable of supportingthe sheet, a flexible impervious membrane for covering said bed, means for applying and maintaining a fluid pressure differential across the bed and membrane to urge the membrane against the bed, means for supporting the membrane away from a sheet on the bed, and means for introducing the desired fluid between the membrane and a sheet on the bed.

4. Apparatus for treating pervious sheets with fluids under pressure comprising a porous bed capable of supporting the sheet, a flexible impervious membrane for covering said bed, said membrane having a hole adapted to overlie a sheet on the bed, means for applying and maintaining a fluid pressure differential across the bed and membrane to urge the membrane against the bed, means for supporting the membrane away from a sheet on the bed, and means for introducing the desired fluid through said hole and between the membrane and a sheet on the bed.

5. Apparatus for treating pervious sheets with fluids under pressure comprising a base having a drain, a flat porous bed carried by the base for supporting a sheet in fixed position, a cover for said bed having fluid inlet means, means interposed betwen said base and said cover at their edges to retain fluids therebetween, a flexible impervious membrane covering said bed, said membrane having a hole communicating with said fluid inlet means, and means for maintaining a fluid pressure differential across the bed and membrane to urge the membrane against the bed.

6. A method of treating a permeable sheet with a fluid on a substantially flat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet-side of said porous surface and the sheet thereon with a flexible impervious membrane, applying a fluid pressure differential across the porous surface and membrane with the porous surface at a pressure less than that above the membrane, and introducing the desired fluid for treating the sheet between the sheet and membrane while the sheet is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

7. A method of treating a permeable sheet with a fluid on a substantially flat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet-side of said porous surface and the sheet thereon with a flexible impervious membrane, applying a fluid pressure differential across the porous surface and membrane with the porous surface at a pressure less than that above the membrane, supporting the membrane from the sheet and introducing the desired fluid for treating the sheet between the sheet and membrane while the sheet is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

8. A method of treating a skin with a fluid on a substantially flat porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin-side of said porous surface and the skin thereon with a flexible impervious membrane, applying a fluid pressure differential across the porous surface and membrane with the porous surface at a pressure less than that above the membrane, and introducing the desired fluid for treating the skin between the skin and membrane while the skin is extended on the porous surface and while maintaining lthe said fluid pressure differential across the membrane and porous surface.

9. A method of treating a skin with a fluid on a substantially flat porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin-side of said porous surface and the skin thereon with a flexible impervious membrane, applying a fluid pressure differential across the porous surface and membrane with the porous surface at a pressure less than that above the membrane, supporting the membrane from the skin and introducing the desired fluid for treating the skin between the skin and membrane while the skin is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

10. A method of treating a permeable sheet with a fluid on a substantially flat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet-side of the porous surface and an area of the sheet inwardly from its marginal edge with a flexible impervious membrane, applying a fluid pressure dierential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, and while the sheet is extended on the porous surface and While the said fluid pressure differential is maintained, flowing the desired fluid for treating the sheet onto the upper surface of the sheet and between the sheet and membrane to supply the fluid to the area of the sheet covered by the membrane.

11. A method of treating a permeable sheet with a uid on a substantially flat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet side of the porous surface and an area of the sheet inwardly from its marginal edge with a exible impervious membrane, applying a fluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane to press the membrane against the porous surface and in contact with the edge of the sheet, and introducing the desired Huid for treating the sheet between the membrane and sheet onto the area of the sheet covered by the membrane without substantially disrupting the contact between the membrane and the edge of the sheet.

12. A method of treating a permeable sheet with a fluid on a substantially Hat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet side of the porous surface and the sheet thereon with a flexible impervious membrane having a hole therein with the hole overlying a central region of the sheet and membrane overlying the area inwardly of the marginal edge of the sheet, applying a fluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, and introducing the desired fluid for treating the sheet through said hole onto the sheet and between the sheet and membrane while the sheet is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

13. A method of treating a permeable sheet with a fluid on a substantially flat porous surface of area larger than that of the sheet, comprising placing the sheet extended on the porous surface, covering the sheet side of the porous surface and the sheet thereon with a exible impervious membrane having a hole therein with the hole overlying a central region of the sheet and mem brane overlying the area inwardly of the marginal edge of the sheet, applying a fluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, supporting the membrane from the sheet, and introducing the desired fluid for treating the sheet through said hole onto the sheet and between the sheet and membrane while the sheet is extended on the porous surface and while maintaining the said uid pressure differential across the membrane and porous surface.

14. A method of treating a permeable skin with a fluid on a substantially flat porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin-side of the porous surface and an area of the skin inwardly from its marginal edge with a flexible imprevious membrane, applying a fluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, and while the skin is extended on the porous surface and while the said uid pressure differential is maintained, flowing the desired uid for treating the skin onto the CII 8 upper surface of the skin and between the skin and membrane to supply the fluid to the area of the skin covered by the membrane.

15. A method of treating a permeable skin with a fluid on a substantially at porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin side of the porous surface and an area of the skin inwardly from its marginal edge with a flexible impervious membrane, applying a fluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane to press the membrane against the porous surface and in contact with the edge of the skin, and introducing the desired fluid for treating the skin between the membrane and skin onto the area of the skin covered by the membrane without substantially disrupting the contact between the membrane and the edge of the sheet.

16. A method of treating a skin with a uid on a substantially flat porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin side of the porous surface and the skin thereon with a flexible impervious membrane having a hole therein with the hole overlying a central region of the skin and membrane overlying the area inwardly of the marginal edge of the skin, applying a uid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, and introducing the desired fluid for treating the skin through said hole onto the skin and between the skin and membrane while the skin is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

17. A method of treating a skin with a uid on a substantially flat porous surface of area larger than that of the skin, comprising placing the skin extended on the porous surface, covering the skin side of the porous surface and the skin thereon with a exible impervious membrane having a hole therein with the hole overlying a central region of the skin and membrane overlying the area inwardly of the marginal edge of the skin, applying a tluid pressure differential across the membrane and porous surface with the porous surface at a pressure less than that above the membrane, supporting the membrane from the skin, and introducing the desired uid for treating the skin through said hole onto the skin and between the skin and membrane While the skin is extended on the porous surface and while maintaining the said fluid pressure differential across the membrane and porous surface.

References Cited in the tile of this patent UNITED STATES PATENTS 119,238 Morris Sept. 26, 1871 1,363,771 Gilardini Dec. 28, 1920 2,307,985 Beasecker lan. 12, 1943 2,354,916 Hurt Aug. l, 1944 2,472,682 Rand lune l, 1949 

1. APPARATUS FOR TREATING PERVIOUS SHEETS WITH FLUIDS UNDER PRESSURE COMPRISING A POROUS BED CAPABLE OF SUPPORTING THE SHEET AND HAVING A SUBSTANTIALLY FLAT UPPER SURFACE, A FLEXIBLE IMPERVIOUS MEMBRANCE FOR COVERING SAID BED, MEANS FOR APPLYING AND MAINTAINING A FLUID PRESSURE DIFFERENTIAL ACROSS THE BED AND MEMBRANE TO URGE THE MEMBRANE AGAINST THE BED, AND MEANS FOR INTRODUCING THE DESIRED FLUID BETWEEN THE MEMBRANE AND A SHEET ON THE BED. 